This invention relates to a spectrometer module, a monitor module, a monitoring unit and a monitoring system for use in monitoring an optical network.
The invention further relates to a spectrometer device, for spectrometry purposes.
The use of optical signals for purposes as carrier of information and the like is currently increasing rapidly. Consequently, there is a current need for developing new methods and products for dealing with this kind of information.
One field of technology is fibre optical communication systems, that are used for transferring large amount of information over large distances. Such system comprise a plurality of different network elements, being interconnected to form a communication network. However, as these networks are becoming larger and larger, and including more and more sophisticated network components, the risk for network faults is increasing rapidly. Consequently, there is a need for surveillance systems, keeping track of the optical signals being transmitted in the network, and reporting when an error has occurred.
Furthermore, the communication speed in the optical networks, such as WDM networks are constantly increasing, resulting in the need of less capacity demanding communication protocols. Thereby, existing protocols such as SDH/SONET, having built-in monitoring functions need to be replaced or at least supplemented. This has lead to the development of systems monitoring data in the optical domain.
One such surveillance or monitoring system is described in the patent document U.S. Pat. No. 6,104,492, in which an apparatus and a method of operating an optical signal monitor for WDM networks is disclosed. Here, a temperature tuned filter is used for generating an optical spectrum monitor for multi-wave signals. The tuneable filter is used for selecting the wavelength that is to be detected and monitored.
However, this technique is rather expensive and also relatively slow as it is based on temperature effects. Other known means of measuring the power spectrum is by use of a diffraction grating, which spatially distributes the wavelengths to one or many detectors. The drawbacks of such technology, however, are that expensive detector arrays is required, that the diffraction efficiency might be rather poor, large demands for careful mechanical alignment and as a result less mechanical robustness.
The proposed means of monitoring the signal quality in WDM systems are usually limited to monitoring of the signal power as a function of wavelength, by use of some spectrometer technology, such as e.g. fibre Bragg gratings, such as for example described in the patent document WO 0102885. This spectrometer technology, however, suffers from the same drawbacks as the ones listed above. An alternative monitoring method is further described in xe2x80x9cQuality monitoring of optical signals influenced by chromatic dispersion in a transmission fibre using averaged Q-factor evaluationxe2x80x9d by I. Shake, H. Takahara, K. Uchiyama, and Y. Yamabayashi, IEEE Photonics Technology Letters, vol 13, pp385-387 (2001). This method, which uses an asynchronous sampling system, is however rather complex and expensive. Yet another proposed method for signal monitoring is described in the document xe2x80x9cOptical Performance Monitoring in Reconfigurable WDM Optical Networks Using Subcarrier Multiplexingxe2x80x9d by G. Rossi, T. E. Dimmick, and D. J. Blumenthal, Journal of Lightwave Technology, vol 18, pp. 1639-1648 (2000). Also this approach, which is based on a sub-carrier modulation of the signal, is complex and expensive.
Consequently, an object of this invention is to achieve a spectrometer module, a monitor module, a monitoring unit and a monitoring system for use in monitoring an optical network, overcoming the above-mentioned drawbacks with the prior art.
A further object is to achieve a way of measuring properties of an optical signal, such as power, state of polarisation and degree of polarisation versus wavelength in an efficient way.
Yet a further object is to achieve a spectrometry device, overcoming the drawbacks with the prior art.
Further objects of this invention are evident from the following description of the invention.
The above and other objects are wholly or partly achieved by a spectrometer module comprising:
an input, for receiving an incoming optical signal,
a variable differential group delay (DGD) element, for applying a variable birefringence retardation to said incoming optical signal,
a detector unit for detecting the power of a defined state of polarisation of a signal exiting said variable DGD element.
By using a variable DGD element in a spectrometer module, it is possible to perform measurements on an incoming optical light signal, for detecting parameters such as polarization state and degree of polarisation as a function of the wavelength of the incoming signal.
In accordance with a first embodiment of the invention, said variable DGD element is implemented spatially. Preferably, said variable DGD element comprises a plurality of laterally spaced sub-elements having different optical lengths, whereby different parts of the incoming optical signal is arranged to be transmitted through different sub-elements of the DGD element. Suitably, said incoming optical signal is arranged to have essentially the same width as said variable DGD element, thereby covering each of said laterally spaced sub elements. Thereby, the DGD element is optimally used. Moreover, said variable DGD element suitably comprises a plane incidence surface, being essentially orthogonal to the optical signal path, and a stepped exit surface.
Alternatively, said variable DGD element may comprise a birefringent element having a decreasing thickness in a direction being transverse to said incoming optical signal. Thereby different parts of the optical signal beam experiences different optical paths. Preferably, said detector unit comprises an array of detectors, and a lens that is placed between said variable DGD element and said detector unit, whereby said detector array is arranged in the Fourier focal plane of said lens. Furthermore, said incoming optical signal is suitably arranged to be slightly divergent.
According to a second embodiment of the invention, said variable DGD element is implemented temporally, by use of an electrical control signal that changes the DGD with time. Preferably, said variable DGD element is comprised by a birefringent system, being essentially sandwiched between a first and a second reflective element, whereby said incoming optical signal is arranged to be reflected between said reflective elements one or more times before outputted from said birefringent system. Suitably, said first and second reflective elements are constituted by a first and second mirror element, respectively. Alternatively, said first and second reflective elements are constituted by a first and a second retroreflector, respectively. By using the above reflections systems, the same birefringent element may by utilized many times.
In accordance with preferred embodiments, a polarizer is arranged between said variable DGD element and said detector unit, said polarizer not being aligned relative to the birefringence eigenaxes of said DGD element. Moreover, said incoming optical signal preferably has a polarisation so as to inject light in both birefringence eigenaxes of said variable DGD element.
Preferably, said variable DGD element is manufactured from an electro-optical birefringent material. Furthermore, said variable DGD element is suitably arranged between a first and a second electrode, said electrodes being arranged to generate an electric field over said variable DGD element. Thereby, a variable DGD element is achieved, being easy to control by means of an electronic control unit or the like. Alternatively, said variable DGD element may be connectable with an acusto-optic transducer, for achieving the above controllability.
Furthermore, said detector unit is preferably connectable with an electronic processing device, in which a detected signal may be processed to extract information regarding properties such as power, state of polarisation and degree of polarisation of said incoming optical signal as a function of wavelength.
The above and other objects of the invention are also achieved wholly or in part by a spectrometer device for measuring the optical spectrum of an optical signal, characterised in that said spectrometer device comprises a first and a second spectrometer module as described in above, and a polarisation splitter, whereby said polarisation splitter is arranged to split said optical signal into a first and a second signal segments, whereby said first signal segment is arranged to be inputted to said first spectrometer module, and said second signal segment is arranged to be inputted to said second spectrometer module. Thereby, the inventive spectrometer module may be used for pure spectrometry, analysing partly or arbitrary polarised light. Such a device may for example be used in the fields of chemistry, process industry, astronomy or pharmaceutical industry, or in any other field in which spectrometry is applied.
Further, the above and other objects are wholly or partly achieved by a monitor module for measuring properties such as power, state of polarisation and degree of polarisation of an incoming optical signal as a function of wavelength, said monitor module comprising:
a polarisation control module being connected with a control unit,
a spectrometer module, being connected with said control unit, and
a polariser being placed between said polarisation control module and spectrometer module.
Preferably, said spectrometer module is as described above. Such a monitor module may for example be used for monitoring the above parameters in an optical network, such as an WDM network. Suitably, said polarisation control module comprises:
a first birefringent element,
a second birefringent element,
each of said birefringent elements being connected with a power source for individual control of the birefringence of said first and second birefringent element, respectively. According with an embodiment, the birefringent eigenaxes of said second birefringent element is rotated by 45 degrees in relation to the birefringent eigenaxes of said first birefringent element. Alternatively, the birefringent eigenaxes of said first and second birefringent elements are coinciding, and a quarter wave element, being rotated by 45 degrees in relation to the birefringent eigenaxes of said birefringent elements, is arranged between said first and second birefringent elements.
The above and other objects are also wholly or partly achieved by a unit for monitoring an optical signal, being transmitted in an optical network, said unit comprising:
a coupler, being arranged to be inserted along a optical transmission path of said optical network, said coupler having a main in- and output, respectively, for receiving and transmitting said optical signal and at least one drop output, to which a portion of said optical signal is droppable, said drop output being connected with one of a spectrometer module as described above and a monitor module as described above. Such a monitoring unit may be installed in an optical network, in a position which is desired to monitor.
Finally, the above and other objects are also wholly or partly achieved by a monitoring system for an optical network, comprising a plurality of network elements, such as transmitters, receivers, transmission lines, amplifiers or the like, said monitoring system comprising:
two or more monitoring stations, each of said monitoring stations being positioned between two network elements of said optical network and each of said stations comprising one of a spectrometer module as described above, a monitor module above and a monitoring unit as described above and
a monitoring hub, being connected with each monitoring station, said hub being arranged to receive measured signal data from each of said monitoring stations, and said monitoring hub comprising a processing unit for processing said measured signal data.
Alternatively, the objects are wholly or partly achieved by a monitoring system for an optical network, comprising a plurality of network elements, such as transmitters, receivers, transmission lines, amplifiers or the like, said monitoring system comprising:
two or more monitoring stations, each of said monitoring stations being positioned between two network elements of said optical network, each of said stations being arranged to measure power, state of polarisation and degree of polarisation of an optical signal entering said monitoring station via said network,
a monitoring hub, being connected with each monitoring station, said hub being arranged to receive measured signal data from each of said monitoring stations, and said monitoring hub comprising a processing unit for processing said measured signal data.
Thereby, the invention may be used for gaining information regarding the function of an entire network, for example enabling quick location of faults.
The above monitoring system as well as the above monitoring module may be used for monitoring signal data quality in a wavelength division multiplexed (WDM) fibre optical communication system. Further, the spectrometer module above may be used as a standalone spectrometer device, as stated above.
Finally, the above objects are wholly or partly achieved by a method of monitoring and measuring properties such as power, state of polarisation and degree of polarisation of an incoming optical signal as a function of wavelength, the method comprising the steps of:
inputting said incoming optical signal to a variable DGD element,
applying a variable birefringence retardation to said incoming optical signal by letting it pass said variable DGD element, and
detecting the power of the signal exiting said variable DGD element, having a determined state of polarisation.
Preferably, said variable DGD element is comprised in a spectrometer module as described above. Moreover, the method suitably comprises the step of dropping said incoming optical signal from a wavelength division multiplexed (WDM) fibre optical communication system that is to be monitored.